In a SMES device, energy in form of a magnetic field can be stored in a coil made of superconducting material where a superconducting current circulates. The stored energy is ½.L.I2 where L is the inductance and I is the current circulating in the coil. Since the coil is superconducting, very small losses are present and the storage time is very long. When energy is required, the current can be redirected and the energy stored in the coil is transferred to work in the coil. Most, if not all, of the present day solutions for charging and discharging a SMES are based on redirecting the current into or out of the coil using switches. Generally the switches are mechanical or solid state or a combination of both and can either be placed in the cooling medium that cools the superconducting coil or placed outside the cooling medium, e.g. at room temperature. However, in both cases, the redirection of the current is based on the principle of breaking the superconducting circuit to force the current to flow in a path through the load. In particular, in such known devices, current leads are present between the cooled superconducting coil and the outside in order to input energy to, or output energy from, the superconducting coil in the form of an electrical current.
The design of such feed-through current-carrying leads is one of the main difficulties when constructing a SMES device. At some point of the feed-through there must be a change from a superconducting conductor to a good room-temperature electrical conductor. However, a good electrical conductor is generally also a very good heat conductor, which makes the thermal isolation of the cold space difficult.
Another drawback with using conventional switches is that the switches always exhibit resistive losses since it is practically impossible to construct a purely superconducting switch. Therefore, conventional SMES devices will suffer from losses even in stand-by mode, which is a serious drawback for long time storage devices.
It is of course known to transfer energy between coils using induction. For example this concept is used with transformers. However, inductive energy transfer is only possible with time varying currents. In both U.S. Pat. No. 4,939,444 and U.S. Pat. No. 5,682,304, SMES apparatuses are disclosed in which coils are inductively coupled together. These known apparatus use oscillation circuits to create oscillating currents. The authors of these known specifications have correctly realised that the superconducting properties of a coil are destroyed if the magnetic field is too high. They refer to the possibility of storing a large amount of energy in the coil since the magnetic field will destroy its superconducting properties. The solution to the problem suggested by these authors is to wind a pair of coils in opposite directions in order to cancel out the magnetic field. However, it is claimed that, since the magnetic field is cancelled out, the current flowing in the superconducting coil arrangement can be increased substantially. It is believed, however, that such a coil arrangement will not store any energy although the current can be increased. This is because the energy is “in” the magnetic field itself. With no magnetic field no magnetic energy is present.
U.S. Pat. No. 5,682,304 does disclose a number of inductive couplings between the coils. However, the couplings between coils 9 and 12 and between coils 22 and 24 are only present in order to cancel out the magnetic field. Moreover, in order to achieve an oscillating circuit formed by components 30 and 33, the coupling factor between coil 36 and coil 30 must not be too good. With perfect coupling the inductance in the LC-circuit is zero. However, with a coupling factor less than unity an energy transfer may be possible since the leak inductance is larger than zero, although this is not what is described.
One aim of the present invention is to provide an improved SMES device of the kind referred to in which there are no mechanical connections connected to the superconducting first coil means for inputting energy to and/or outputting energy from the first coil means.
Another aim of the present invention is to enable energy to be inputted to or partially extracted from stored energy of superconducting coil means.
According to one aspect of the present invention there is provided a superconducting magnetic energy storage (SMES) device comprising a first coil means made of superconducting material, cooling means for cooling the first coil to superconducting temperatures, second coil means inductively coupled to the first coil means for inputting energy to, and/or outputting energy from the first coil means, and switching means for switching the first coil means between a superconducting condition and a non-superconducting condition, characterised in that the first coil means is arranged as a closed loop electric circuit having no connecting means mechanically connected thereto for inputting or outputting energy thereto, and in that the switching means comprises a third coil means for the application or removal of a magnetic field for switching the first coil means between its non-superconducting and superconducting conditions.
The invention is based on the concept of transferring energy between the superconducting (cryogenic) first coil means to the normally conducting second coil means at an elevated (relative to cryogenic) temperature, e.g. at room temperature, without using an oscillating circuit. The required time varying current is for inductively transferring energy is obtained by making the superconducting material of the first coil means normal-conducting with a large enough magnetic field from a the third coil means, preferably arranged perpendicular to the other two coil means. As long as this quench-field is applied the current in the first coil means decays. According to Faraday's law of induction the second coil means tries to oppose this change by inducing a current in the same direction. This current is used to provide a load with power. In practice a capacitor is charged to a desired voltage which, in turn, is discharged in a controlled way over a load. (If the load requires an ac-current there must be a dc-ac converter between the capacitor and the load.)
The present invention is based on experimental results proving that it is possible to partially or completely discharge a closed-loop superconductor in the manner described above. This opens the possibilities to isolate the cryogenic parts from the parts that can or must be at an elevated temperature, e.g. at room temperature, in a SMES application. The main consequence of this is that the heat leakage into the cryogenic parts can be minimised. Furthermore, a bulk cylinder of the superconducting material can be used as a one-turn first coil means. Since the first coil does not have switches or other components physically attached to it, the resistive losses are minimised.
Although not essential, it is preferred that the first coil means comprises a single turn, e.g. in the form of a cylinder. Alternatively, for example, the first coil means may be of other shapes, e.g. of toroidal form.
Preferably the switching means further comprises control means for controlling the current supplied to the third coil means. In this way the amount of quenching of the superconducting first coil means can be controlled. Suitably, the control means comprises a current pulse generator for applying control current pulses to the third coil means. Conveniently the pulse generator is able to control the amplitude and/or duration of the pulses to control the magnetic field applied by the third coil means.
Conveniently the first coil means can be made of any kind (high or low-temperature SC) and of any form (wires, bulk) of superconducting material. Preferably, however, the first coil means is made of a bulk ceramic high-temperature superconducting material, such as YBCO or BSCCO, preferably arranged in a single turn. Preferably the superconducting material should be anisotropic such that the maximum allowed current in the direction of the axis (the “c-axis”) of the first coil means is much less than the critical current in the plane (the “a-b plane”) perpendicular to the c-axis. The reason for this is that it is necessary to make the superconducting first coil means normally conducting (i.e. non-superconducting) in order to charge and discharge the SMES. This is achieved by applying a field-pulse in the a-b plane which is sufficiently large such that the super-conducting material is transferred to its normal conducting state. The more anisotropic the superconducting material is the smaller amplitude of the pulse that is required.
Suitably the cooling means comprises a cryogenic container, e.g. a dewar, in which the first coil means is situated. The second coil means is preferably arranged outside the cryogenic container. By suitable shaping of the cryogenic container, the third coil means may also be situated outside the cryogenic container. For example, in the case of the first coil means being in the form of a cylinder, the cryogenic container may have an annular form with the third coil means surrounding a part of the annular cryogenic container.
Preferably the third coil means is arranged to supply a magnetic field in a plane substantially perpendicular to the main (cylindrical) axis of the first coil means.
Preferably the first and second coils are coaxial with each other.
According to another aspect of the present invention there is provided a method of inputting energy to and/or outputting energy from a first coil means made of superconducting material and cooled to superconducting temperatures, comprising inductively coupling second coil means to the first coil means, wherein the first coil means is arranged in a closed electrical circuit with no mechanical connections thereto for inputting or outputting energy to the first coil means and wherein the first coil means is rendered non-superconducting or superconducting by the application or removal of a magnetic field via a third coil means.
Preferably the magnitude of the magnetic field applied by the third coil means is controllable. By applying a controllable current, e.g. via current pulse control, the first coil means can be partially quenched so that energy can be partially extracted from or supplied to the first coil means.
According to a still further aspect of the present invention there is provided a power supply system including a magnetic energy storage device according to said one aspect of the present invention. The power supply system may be able to provide energy storage, power quality, peak load security and/or power supply security. Power quality, peak loads and power supply security may be specified closely and numerically in typical contracts for delivery and maintenance of electrical power by a power supply system.